The challenges of bonding in light-weighting - and how adhesives turn bonding into a technology enabler

The importance of light-weighting is well known across a wide variety of design and assembly applications, including transportation, industrial processes, even wheelchairs. The benefits are indisputable: reduced energy consumption, environmental considerations, more flexible product design, better performance and potentially reduced end-user costs.

In light-weighting, clearly every gram counts. However, while the need to use materials that weigh as little is possible is well understood – such as plastics, foams and light metals - less known is the impact that the choice of bonding method or material can make. These range from the obvious, such as the fact that fixers and fastenings immediately add unwanted grams, but beyond added weight, there are other problems that traditional bonding approaches can introduce.

Let’s examine each of the main challenges of bonding in lightweight design and how to address each of those using modern adhesives, which are going beyond just problem-solving and is enabling more innovative designs and assembly processes.

Bonding dissimilar materials

Lightweight designs often involve bonding of two dissimilar substrates, such as a plastic and a lightweight metal. These may have very different inherent weights, as well as different levels of surface energy, so immediately there is the issue of bonding two different substrates, while still needing to preserve their structural integrity and load balances evenly across the surface area to prevent stress concentrations.

So, what might that mean in practice? Clearly, it is not easy to weld plastics to metals, so that process is out of the window straight away. Using rivets or other fasteners adds weight, while holes create weaknesses. Adhesives, on the other hand, not only typically weigh less when compared to other types of bonding, they also enable even load balances and – when choosing the right adhesive – are ideal for bonding disparate materials.

Simultaneously bonding and sealing

When the aim is to create a secure seal as well as a bond, then it is vital that the choice of bonding material does not create further problems, such as drilling holes that lead to gaps or areas of exposure, which in turn can lead to moisture ingress or corrosion, airflow or heat intrusion.

Adhesives are increasingly being used to both connect substrates and seal surface areas, because of their abilities to be both watertight and airtight, while still providing a strong, lightweight bond. For instance, liquid adhesives can flow into even the smallest of spaces to provide a secure seal.

Noise, vibration and harshness (NVH)

The industries where light-weighting is being used – let’s take motor vehicles as a good example – are typically also ones where designers want to remove all those rattles, vibration from the wind, engine or fan noise, judders, jerks and shakes. Not only are these all very annoying, they represent wasted energy and over time could cause wear-and-tear on components.

Once again, adhesives have a dual-role to play: as well as lightweight bonding, soft or malleable adhesives and tapes can be used to absorb energy.

Thin substrates

Where thin substrates are used, drilling holes or applying heat to achieve a bond can denigrate the surface, such as causing distortion, warping or weakening. Drilling also involves adding an extra assembly process and therefore additional time and cost, as well as unwanted weight which can also affect a thin substrate.

Thanks to the latest advances in liquid adhesive and tape innovation, it is now possible to bond even the thinnest of substrates, without any compromise over design or performance.

Thermal expansion capability

Its ‘engineering 101’: materials expand or contract according to the ambient temperature, plus different substrate and components do so at their individual rates. As well as natural variants in temperature, there is also the additional heat created by, for instance, a high-speed vehicle going around a Formula One track.

The problem with traditional bonding is that expansion or contraction can lead to all kinds of problems, such as allowing unwanted movement between parts, letting in moisture or air, even product failure.

The answer? Choose a liquid adhesive or tape that is able to bond surfaces with different co-efficient of expansion and contraction, without any degradation of bonding performance.

Aesthetics

Last but not least, there is the aesthetic consideration: in many light-weighting applications, especially consumer-facing products, looks matter. Obviously, bolts, holes and other traditional methods of bonding have to be incorporated into the visual design and arguably, can often detract from overall appearance.

Adhesives, however, are largely invisible. As they are ‘inside’, they remove from sight join lines, any fixing points. Plus, they can be the catalyst for more imaginative, flexible design concepts, providing far more freedom to innovate than ever before.

Choosing the right adhesive for light-weighting

So, adhesives are the answer to all light-weighting bonding applications?

Not exactly, because the use of adhesives brings their own set of considerations. Using adhesives in light-weighting design is not difficult, just different and any engineer can easily arm him or herself with knowledge of the basics. It comes down to understanding the stresses and loads of different adhesives, not just the substrates they are bonding.

For instance, Polytetrafluoroethylene (PTFE) materials and most lower-density polymers have lower surface energy substrates, making them trickier to bond. This can lead to surface preparation steps being required. In the case of some substrates, such as syntactic foam which is often used in marine light-weighting, the wrong adhesive might lead to a risk of ‘read through’.

The good news is that adhesive technology has progressed in leaps and bounds in recent years and 3M has developed a range of solutions that addresses most light-weighting bonding challenges imaginable, across different modulus, viscosity, thermal expansion rates, flexibility and ability to bond different substrates. Adhesives are delivered in different formats, including sprays, tapes and acrylics.

The range includes adhesives that are highly effective without needing surface preparation to achieve the necessary level of wet-out to achieve a robust bond (in other words, the process by which contact and attraction of two substrates is maximised). For example, 3M has a special adhesive that structurally bond polyethylene or polypropylene without surface pre-treatment.

Pushing light-weighting boundaries with adhesives

With light-weighting continuing to be a dominant trend in product design, the importance of choosing the best possible bonding technique will increasingly play a more central role as designers and manufacturers try to remove as much unnecessary weight as possible. The good news is that adhesives not only step up to the challenge, they turn bonding from being just another process to an enabling technology, one that can help companies of all kinds to continue to push light-weighting boundaries, now and for years to come.